1. Field of the Invention
The present invention relates to a three-lens-unit zoom optical system used in a camera for photography, and particularly in a lens shutter camera.
2. Description of Related Art
Recently, in lens shutter cameras, those provided with zoom lenses have been popularized and those mounting photographic lenses which are compact and show high variable magnification ratios have come to be required. For a variable magnification ratio of approximately 2-3, a zoom lens constructed with two lens units having positive and negative refracting powers, arranged in this order from the object side, is generally used. As for a variable magnification ratio of approximately 3-4.5, a zoom lens constructed with three lens units having positive, positive, and negative refracting powers, arranged in this order from the object side, is generally used. However, in the zoom lens constructed with the three lens units having positive, positive, and negative refracting powers in which the variable magnification ratio is above 3, the combined refracting power of a first lens unit with positive power and a second lens unit with positive power must be increased at the wide-angle position of the zoom optical system in an attempt to obtain a wide angle of a certain extent. In particular, to suppress the production of aberration in the second lens unit, an increase of the number of lenses is indispensable.
In order to obtain good performance in the entire zoom area extending from the wide-angle position to the telephoto position, it is necessary to reduce the amount of aberration in each of the zoom lens units. Since in particular the amount of aberration of a third lens unit is increased at the telephoto position, at least two lenses are required for the third lens unit.
Thus, in an attempt to achieve a wide-angle design with the three lens units constructed as mentioned above, there is the need to enlarge the number of lenses of the second and third lens units and to increase the entire lens length along the optical axis. As a result, it is difficult to reduce the entire lens length where the lens barrel of a camera is collapsed.
Conventional techniques attempting to solve such problems are disclosed in Japanese Patent Kokai Nos. Hei 11-223771, Hei 11-142741, Hei 10-307259, Hei 7-43612, Hei 5-264903, and Hei 11-295600.
In each of the techniques described in Kokai Nos. Hei 11-223771, Hei 11-142741, and Hei 10-307259, the number of lenses is lessened and compactness is achieved. However, the variable magnification ratio is less than 2 and a high variable magnification ratio is not obtained.
In each of the techniques described in Kokai Nos. Hei 7-43612 and Hei 5-264903, a variable magnification ratio of more than 3 is achieved, but compactness is not obtained because of a large number of lenses.
In the technique described in Kokai No. Hei 11-295600, aspherical surfaces are effectively used and thereby a reduction in the number of lenses and an increase in the variable magnification ratio are attained. However, the aspherical surfaces are used for both sides of the last glass lens which is largest in diameter. This makes the manufacture of the lens difficult and results in a considerable increase of cost, which is unfavorable.
It is, therefore, an object of the present invention to provide a three-lens-unit zoom optical system in which a variable magnification ratio of more than 3 is achieved and a wide-angle design and good imaging performance are obtained. In order to accomplish the above object, the three-lens-unit zoom optical system in a first aspect of the present invention includes, in order from the object side, a first lens unit with positive refracting power, a second lens unit with positive refracting power, and a third lens unit with negative refracting power. These individual lens units are moved toward the object side so that when the magnification of the zoom optical system is changed, extending from the wide-angle position to the telephoto position, a space between the first lens unit and the second lens unit is widened and a space between the second lens unit and the third lens unit is narrowed. In this case, the second lens unit has at least one negative lens and at least two positive lenses, and the third lens unit has, in order from the object side, a lens with at least one aspherical surface and a negative lens, satisfying the following conditions:
0.5 less than |f3|/h less than 0.8xe2x80x83xe2x80x83(1) 
xe2x88x920.2 less than f3/f31 less than 0.2xe2x80x83xe2x80x83(2) 
where f3 is the focal length of the third lens unit, h is the maximum image height, and f31 is the focal length of the lens with at least one aspherical surface in the third lens unit.
The three-lens-unit zoom optical system in a second aspect of the present invention includes, in order from the object side, a first lens unit with positive refracting power, a second lens unit with positive refracting power, and a third lens unit with negative refracting power. These individual lens units are moved toward the object side so that when the magnification of the zoom optical system is changed, extending from the wide-angle position to the telephoto position, a space between the first lens unit and the second lens unit is widened and a space between the second lens unit and the third lens unit is narrowed. In this case, the third lens unit has, in order from the object side, a plastic lens with at least one aspherical surface and a negative lens, satisfying Condition (1).
The three-lens-unit zoom optical system in a third aspect of the present invention includes, in order from the object side, a first lens unit with positive refracting power, a second lens unit with positive refracting power, and a third lens unit with negative refracting power. These individual lens units are moved toward the object side so that when the magnification of the zoom optical system is changed, extending from the wide-angle position to the telephoto position, a space between the first lens unit and the second lens unit is widened and a space between the second lens unit and the third lens unit is narrowed. In this case, the second lens unit has, in order from the object side, a negative lens, a positive lens, and a positive lens, and the third lens unit has, in order from the object side, a lens with at least one aspherical surface and a negative lens, satisfying Condition (2) and the following condition:
1.2 less than fw/h less than 1.6xe2x80x83xe2x80x83(3) 
where fw is a focal length at the wide-angle position.
The three-lens-unit zoom optical system in a fourth aspect of the present invention includes, in order from the object side, a first lens unit with positive refracting power, a second lens unit with positive refracting power, and a third lens unit with negative refracting power. These individual lens units are moved toward the object side so that when the magnification of the zoom optical system is changed, extending from the wide-angle position to the telephoto position, a space between the first lens unit and the second lens unit is widened and a space between the second lens unit and the third lens unit is narrowed. In this case, the third lens unit has, in order from the object side, a lens with at least one aspherical surface and a negative lens, satisfying Conditions (1) and (2) and the following condition:
0.3 less than fw/f1 less than 0.6xe2x80x83xe2x80x83(4) 
where f1 is the focal length of the first lens unit.
The three-lens-unit zoom optical system in a fifth aspect of the present invention includes, in order from the object side, a first lens unit with positive refracting power, a second lens unit with positive refracting power, and a third lens unit with negative refracting power. These individual lens units are moved toward the object side so that when the magnification of the zoom optical system is changed, extending from the wide-angle position to the telephoto position, a space between the first lens unit and the second lens unit is widened and a space between the second lens unit and the third lens unit is narrowed. In this case, the third lens unit has, in order from the object side, a lens with at least one aspherical surface and a negative lens, satisfying Conditions (1) and (2) and the following condition:
0.3 less than DG2/f2 less than 0.6xe2x80x83xe2x80x83(6) 
where DG2 is a distance along the optical axis from the most object-side surface of the second lens unit to the most image-side surface and f2 is the focal length of the second lens unit.
This and other objects as well as the feature and advantages of the present invention will become apparent from the following detailed description of the preferred embodiments when taken in conjunction with the accompanying drawings.